Heat seal adhesives for high moisture food and condiments have traditionally been partially crystalline polymers applied from solvent based solutions to the lidding material. The lidding material (often in the form of a film or foil) with applied adhesive is stored until needed to package food or condiments. It is desirable that the adhesive as applied to the lidding material does not adhere to any adjacent/contacting surfaces, e.g. other portions of lidding (adjacent lidding, etc.) while stored in warm or hot warehouses awaiting use (this is referred to as having blocking resistance).
Often, the food or condiments are added to a plastic container on a filling line, a lidding material is cut to a shape consistent with the top of the plastic container and positioned over the container, the lidding and container are brought together in the presence of a heated platen (the platen typically applied to an outside surface of the lidding and shaped to match the shape and dimensions of the top of the container). The heated platen melts and activates the adhesive (between the lidding and the container) to secure the lidding to the container and seal the package thereby preventing both ingression and egression of liquid and solid materials through the adhered surfaces.
A heat seal adhesive for high moisture foods needs to have low moisture sensitivity so it does not swell, whiten or otherwise produce a weakened bond between the container and lidding during storage of the food or condiment. The adhesive desirably has a food contact safe composition that can be approved by appropriate governmental agencies as the food or condiment in a sealed container can come into direct contact with the adhesive on the lidding material during shipping, sale, and post-sale.
When a consumer opens a heat sealed package of food or condiment, they generally prefer that the lidding with the adhesive attached thereto cleanly delaminate from the plastic container leaving the food or condiment in a clean adhesive free container.
U.S. Pat. No. 6,368,707 discloses a polymer for a heat-sensitive adhesive sheet comprising a substrate having formed thereon an adhesive layer comprising a polymer “A” having a glass transition temperature of −10° C. or below obtained by emulsion-polymerizing alkyl(meth)acrylates and copolymer “B” having a glass transition temperature of 20° C. or more and being a water-soluble or water-dispersible copolymer from unsaturated carboxylic acid with other radically polymerizable monomers, wherein part of the entire amount of carboxyl groups in the copolymer are neutralized with a base having a boiling point of 110° C. or below.
U.S. Pat. No. 5,385,967 discloses aqueous dispersions for heat-seal applications with a copolymer A having a glass transition of 50-150° C. and copolymer B with a glass transition of −50 to 50° C. and at least one of the two copolymers containing from 3 to 70% by wt. of an ethylenically unsaturated C3-C5 mono- or dicarboxylic acid or the anhydride thereof. The examples of the patent generally showed the high glass transition temperature polymer with high amounts of ethylenically unsaturated C3-C5 mono- or dicarboxylic acid or the anhydride thereof. Such high glass transition temperature polymers with high amounts of carboxylic resins are generally known as support resins and while providing good colloidal stability tend to impart unacceptable moisture sensitivity in applications where exposure to moisture occurs.
U.S. Pat. No. 5,306,743 discloses an aqueous synthetic resin dispersion comprising latex particles having an average diameter below 145 or 100 nm consisting of a core material with a glass transition temperature above 60° C. and a shell material with a glass transition temperature below 80° C., provided that the glass transition temperature of the shell is at least 20 K below that of the core (indicating a softer shell and harder core).
U.S. Pat. No. 5,837,089 to Ashland discloses a method of using a heat activatable adhesive for laminating vinyl sheet to wood. The organic solvent-free adhesive combines a reactive styrene-acrylic latex emulsion, an ethylene vinyl acetate copolymer emulsion, and a wax. After application to the vinyl substrate the adhesive is heat activated at temperatures as low as 160° F. (71° C.) and the laminate resists delamination at 190° F. (88° C.).
U.S. Pat. No. 7,700,703 to Rohm and Haas discloses a polymeric additive including one or more ethylenically unsaturated surfactant monomers. The additive is described as having a number average molecular weight less than 40,000 and having an improved balance of tack, adhesion, plasticity and peel strength and water whitening resistance as compared to other polymer additives.
U.S. Pat. No. 8,383,723 to Dow discloses method for forming a heat sealable coating on a substrate, wherein the substrate is formed from at least one oriented polymer. The method includes depositing an aqueous polymer dispersion on the substrate, wherein the aqueous polymer dispersion includes (A) at least one thermoplastic resin; (B) at least one dispersing agent; and (C) water; wherein the dispersion has a pH of less than 12, and drying the dispersion to form a first layer. Preferred polymers include copolymers of ethylene and another monomer, such as vinyl acetate, or propylene and another monomer.
WO 2011/017388 to Lubrizol Advanced Materials, Inc. discloses a heat sealable adhesive composition for plastic and metal containers for moderate to high moisture food products. The adhesive comprises latex in water with a minimum film formation temperature near or below 25° C., blocking resistance up to 50° C., heat seal-ability under reasonable conditions, good adhesive properties, and approval for food contact. The latex of WO 2011/017388 is similar to the acrylic latex of this invention.
U.S. Pat. No. 7,189,461 to Air Products discloses an invention directed to aqueous-based ethylene-vinyl acetate (EVA) polymer emulsions suited for use in heat seal applications. The EVA polymers of this invention are produced by a relatively low-pressure process, i.e., less than 2000 psig, preferably from about 1000 to about 2000 psig. The aqueous-based EVA based polymer emulsions contain crystalline segments resulting from ethylene linkages and are prepared by emulsion polymerizing ethylene and vinyl acetate, preferably with a carboxylic monomer, in the presence of a stabilizing system containing surfactant or a cellulosic protective colloid in combination with surfactant.
Heat seal adhesive is a broad category of materials. Some heat seal adhesives (called hot-melt adhesives) are applied as a hot fluid adhesive to one or more substrates to thereafter quickly form an adhesive bond between two or more substrates as the adhesive cools. This adhesive type does not need blocking resistance as the adhesive typically is heated immediately before application to two or more substrates and is not stored in contact with several substrates where it may prematurely cause an adhesive bond. A subset of adhesives for seals is those products where an adhesive is applied to one substrate (typically cardboard, paper, or plastic packaging) and the adhesive coated substrate is further processed or stored. Later during final assembly the adhesive coated substrate is heated above an activation temperature, at which temperature the heat seal adhesive then forms a bond between the original substrate and one or more additional substrates. In many cardboard, paper and plastic adhesive uses, the adhesive can have moisture sensitive components without having adhesion adversely affected (because they are used in relatively dry environments where the moisture effect on the adhesive is minimal).